Thin film mirrors are used to give large field of view displays in image display systems giving displays for simulation, leisure, visualisation or computer design. In the displays for simulation, the thin film mirrors are typically used as part of a collimated or near collimated off-axis display.
The geometric accuracy of the thin film mirror is a major factor in determining the quality, and hence the usefulness of such a mirror, in an image display system. The thin film mirror is manufactured on a suction chamber. The geometric accuracy of the thin film mirror is dependent on the accuracy of edges of the suction chamber, the method of application of the thin film to the edges of the suction chamber, and the deformation of the thin film when differential pressures are applied to either side of the thin film.
The thin film is initially a sheet, the shape of which is part of the frustum of a cone laid flat. The thin film is applied with no initial tension to the suction chamber. Tension in the thin film is introduced when suction or differential pressure is applied and the thin film is drawn back into the suction chamber. An ideal shape will be achieved when the tensions in the thin film are close to uniform over the entire surface. Tensions at any point in the thin film can be considered as a combination of a vertical component and a horizontal component. Vertical tensions can be seen to be uniform, since at all points around the suction chamber, the thin film transitions from a chord to an arc as the thin film is drawn back into the suction chamber. Horizontal tensions however are at a maximum in the middle of the final shape, falling to zero at the top attachment edge and the bottom attachment edge. The mechanical properties of the thin film must be considered. When the thin film is stretched beyond its yield point, the increase in force required to continue stretching significantly reduces. This means that, from the centre of the mirror moving out to the top and bottom edges, there is a large area of the mirror where the horizontal tensions are near to uniform, because the thin plastics film has been stretched beyond its yield point. In some areas of the thin film, a band at the top of the thin film and a band at the bottom of the thin film are areas where the thin film has not been stretched to its yield point, as the horizontal tension reduced rapidly towards the top edge and towards the bottom edge. This lack of tension in the thin film close to and parallel to the top and bottom forming edges of the suction chamber, allows the thin film to move back, under vacuum, substantially more than the ideal vertical profile required. The result is a flatter/longer vertical radius in the central part of the mirror that transitions to a sharper/shorter radius closer to the top and bottom edges. The geometry of the thin film when under tension therefore does not match the desired geometry in the region close to the top of the thin film and in the region close to the bottom of the thin film. These areas introduce unacceptable distortions into any image and are usually masked. These unusable regions are known as dead bands.